Objective In order to discuss the genesis of the lead-zinc deposits in Hengyang Basin and its surrounding areas, this paper has carried out work on the highly studied Qingshuitang lead-zinc deposit.

Methods By means of geological survey, fluid inclusion study of quartz, calcite and sphalerite at different stages and Rb-Sr dating of quartz, we discuss the geochemical characteristics, evolution and mineralization processes of ore-forming fluid.

Results Based on the mineral assemblages and the intercutting relationships between veins, this paper classifies the metallogenesis of the Qingshuitang lead-zinc deposit into three stages: pyrite-quartz stage (Ⅰ), quartz-galena-sphalerite stage (Ⅱ) and barite-calcite stage(Ⅲ). The research of fluid inclusions show that both L- and VL-type primary and secondary fluid inclusions occur in quartzs and sphalerites at stage Ⅱ; VL-type primary fluid inclusions predominantly occurs in quartzs at stage Ⅰ and calcites at stage Ⅲ. The measured temperature results show that primary inclusions of quartz formed at stage Ⅱ can be subdivided into four groups according to the homogeneous temperature and salinity, which are 258-296℃ and 8.55%-9.21% NaCl eqv, 260-298℃ and 3.61%-4.18% NaCl eqv, 120-160℃ and 12.73%-18.22% NaCl eqv, 145-168℃ and 3.06%-3.87% NaCl eqv, respectively. By comparison, the homogenization temperature and salinity of sphalerite are mainly concentrated in the range of 102-178℃ and 10.24%-19.45%NaCl eqv. The ore-forming fluid belongs to the NaCl-H₂O hydrothermal system of medium-low temperature and medium salinity. Based on occurrence and homogenization temperature of inclusions, the medium and low salinity fluids at stage Ⅱ may have different sources and evolutionary processes. The former is mostly distributed in groups and might derive from medium temperature (210-312℃) and medium salinity (12.30-19.30%NaCl eqv) fluid at stage Ⅰ. The latter mostly occurs along mineral fissures, which may be formed by mixing of the late low salinity atmospheric precipitations. In this paper, two quartz Rb-Sr isochron ages are obtained, and they are (88.8±2.4) Ma and (17.86±0.42) Ma, respectively. The former age represents the mineralization age of Qingshuitang lead-zinc deposit, and the latter age records the time of late tectonic superposition. Both of them are significantly later than the diagenetic ages of the Zhoujialing granite ((203.0±1.4) Ma) near the mine site and the adjacent Guandimiao pluton ((223.4±1.9) Ma).

Conclusions Considering that the metallogenic fluids of lead-zinc in the area are related to magmatic activities, it is inferred that there are the Late Cretaceous concealed intrusions in the deep part of the mining area combined with sulfur and lead isotope characteristics of sulfides from the Qingshuitang and Liushutang mining areas, which provide material and energy sources for lead-zinc mineralization.